Categories: Shaft Alignment

Broken Washers Can Cause Headaches!

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By on October 11, 2013

Recently a customer contacted us about increased vibration on a motor, due to a broken washer.  And they wanted to know if there was a specification for washer hardness, and a guide for bolt size to washer size.

There are many standards for bolting (USS, SAE, ASME, etc.), of which, washers are an important part.  As an example, a structure like a bridge or skyscraper may have a stringent metallurgy requirement, while one for a small industrial motor may not.  In industry, the problem is that most purchasing departments specify “washers” – without regard to strength, hardness, malleability, etc.

I have seen broken washers increase machine vibration, since it means that the bolt is now loose.  My opinion, and it is only that, is that broken washers are caused by three things:

  They are cheap

  They are improperly sized or installed, including improper sizing, excessive torque, or installing bolts much smaller than the bolt hole it goes in.

  They are deformed or worn, and should be replaced

Washers should be specified, not just purchased because they are the cheapest.  Hardened washers are available, and can be beneficial.  But increasing the hardness of metal normally increases it brittleness as well. Using two standard flat washers may help.  Your maintenance or design engineer should be able to help determine the proper application.

Washers are sized.  A 3/4″ bolt should use 3/4″ washers.  A 1″ washer fits easily on a 3/4″ bolt, but is too big to do its job, which is to:

  provide a flat surface against which the head of the bolt and the face of the washer contact.

  Increase the load area of the bolt.

  Span the hole diameter.

  Prevent damage to the machine foot by the turning bolt/nut.

 Washers should normally be installed in this fashion.

In addition, washers which are visibly damaged, deformed, cupped, or rusted should not be used.


About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

3 responses to “Broken Washers Can Cause Headaches!”

  1. Themis K. says:

    Once again a really good article with useful info!

  2. jorge says:

    I have a question Stan: How do I know the size of a hold down bolt in order to properly hold the motor? Is there any formula I can use to slects its diameter? Should I only select it based on the electric motor hole diameter?
    I have a case for example with a 16 mm electric motor hole using an 8 mm diameter hold down bolt.

  3. Stan Riddle says:

    Good question Jorge! In the US we use NEMA specs to specify motor dimensions. Most motor holes (or slots) are given as 1/32 inch increments. As an example, 27/32″ motor hole could accept a 3/4 inch bolt, and leave enough gap for alignment.

    Your example of an 8mm bolt in a 16mm hole sounds excessive. I would talk to your engineering department, or contect the motor manufacturer.